There has been substantial interest in exploiting compound semiconductors having particle dimensions on the order of 2-50 nm, often referred to as quantum dots (QDs), nanoparticles, and/or nanocrystals. These materials have high commercial interest due to their size-tunable electronic properties, which can be exploited in a broad range of commercial applications. Such applications include optical and electronic devices, biological labeling, photovoltaics, catalysis, biological imaging, light emitting diodes (LEDs), general space lighting, and electroluminescent displays.
Well-known QDs are nanoparticles of metal chalcogenides (e.g, CdSe or ZnS). Less studied nanoparticles include III-V materials, such as InP, and including compositionally graded and alloyed dots. QDs typically range from 2 to 10 nanometers in diameter (about the width of 50 atoms), but may be larger, for example up to about 100 nanometers. Because of their small size, QDs display unique optical and electrical properties that are different in character to those of the corresponding bulk material. The most immediately apparent optical property is the emission of photons under excitation. The wavelength of these photon emissions depends on the size of the QD.
The ability to precisely control QD size enables a manufacturer to determine the wavelength of its emission, which in turn determines the color of light the human eye perceives. QDs may therefore be “tuned” during production to emit a desired light color. The ability to control or “tune” the emission from the QD by changing its core size is called the “size quantization effect”. The smaller the QD, the higher the energy, i.e. the more “blue” its emission. Likewise, larger QDs emit light more toward the electromagnetic spectrum's red end. QDs may even be tuned beyond visible spectrum, into the infrared or ultra-violet bands. Once synthesized, QDs are typically either in powder or solution form.
A particularly attractive application for QDs is in the development of next generation LEDs. LEDs are becoming increasingly important in modern day life and it's predicted that they have the potential to become a major target for QD applications. QDs can enhance LEDs in a number of areas, including automobile lighting, traffic signals, general lighting, liquid crystal display (LCD) backlight units (BLUs), and display screens.
Currently, LED devices are typically made from inorganic solid-state compound semiconductors, such as GaN (blue), AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). Each of these materials emit a single color of light, as indicated. As white light is a mixture of colors in the spectrum, solid-state LEDs that emit white light cannot be produced using a single solid-state material. Moreover, it is difficult to produce “pure” colors by combining solid-state LEDs that emit at different frequencies. At present, the primary method of producing white light or a mixture of colors from a single LED is to “down-convert” light emitted from the LED using a phosphorescent material on top of the solid-state LED. In such a configuration, the light from the LED (the “primary light”) is absorbed by the phosphorescent material and re-emitted at a second, lower frequency (the “secondary light”). In other words, the phosphorescent materials down-converts the primary light to secondary light. The total light emitted from the system is a combination of the primary and secondary light. White LEDs produced by phosphor down-conversion cost less and are simpler to fabricate than combinations of solid-state red-green-blue LEDs. Unfortunately, however, conventional phosphor technology produces light with poor color rendering (i.e. a color rendering index (CRI)<75).
QDs are a promising alternative to conventional phosphor technology. Their emission wavelength can be tuned by manipulating nanoparticle size. Also, so long as the QDs are monodispersed, they exhibit strong absorption properties, narrow emission bandwidth, and low scattering. Rudimentary QD-based light-emitting devices have been manufactured by embedding coloidally produced QDs in an optically transparent (or sufficiently transparent) LED encapsulation medium, such a silicone or an acrylate, which is then placed on top of a solid-state LED. Thus, the light produced from the LED package is a combination of the LED primary light and the secondary light emitted from the QD material.
However, such systems are complicated by the nature of current LED encapsulants. For example, QDs can agglomerate when formulated into current LED encapsulants, thereby reducing their optical performance. Furthermore, even after the QDs have been incorporated into the LED encapsulant, oxygen can still migrate through the encapsulant to the surfaces of the QDs, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
Thus, there is need in the art for a fast and inexpensive method that can reliably down-convert an LED.